Editor’s note: Colorado author Dimity McDowell wrote “Run Like a Mother” with running partner Sarah Bowen Shea. This excerpt is adapted from the book.

I always liked my exercise straight-up and sweaty: running, preferably, but I’d also take a StairMaster, spinning class or stationary bike. The sign of a good workout was a sports bra I had to wring out immediately in the locker room, not triceps that were tender to the touch 48 hours post-workout.

That instant-gratification mentality began to erode, though, after I had my second kid. At the end of the day, I regularly noticed the indent of my belt on my “abs,” which oozed over my jeans like a melting ice-cream cone. Every time I had to squeeze into a pair of dress pants — not often, thank goodness — I had to resurrect the life-sucking girdle-y thing I wore on my wedding day. My lower back, never one to edit its pain, was, in fact, tender to the touch, but not from any workout.

So I staged my own intervention. With a marathon story assignment, my co-author Sarah and I were fortunate enough to be given coaches. I told Ivana, my coach, that besides crossing the finish line with pride and limbs intact, my next priority was getting strong. Or, as I have explained in countless magazine stories, I would have a hard time going the distance.

Here’s why: the core, an area that runs from the bottom of your ribs to the bottom of your bum, is the metaphorical mother of your body. Her strength dictates your overall strength.

If she’s weak and loosey-goosey, there is no discipline in your body. Your legs overstride, your arms splay like a sprinkler, and you likely end up injured (or at least frustrated because you never get faster).

If she keeps a tight ship and barks out multiple orders at once — instructing your feet to turnover quickly and lightly, your lower back muscles to stabilize, your arms to swing tightly — the better runner you’ll be.

If she has to focus on one weak part of the body, like your right hip, bribing it with TLC so it will move along, some other part, like your left glute, will disobey and flare up.

The situation is not unlike bribing an ornery 3-year-old with fruit snacks, who tells an otherwise content 6-year-old that it was the last pack, who then throws a fit.

Two babies out, I needed a happy, solid core that would support the distance (and I definitely didn’t mind the idea of being a bit sleeker). My routine for the marathon was very simple: six to eight exercises which required either a stability ball or only body weight, and, as such, could be done at home.

I lied. The routine itself was simple; getting it done was anything but. A 20-minute strength routine, interrupted by Amelia asking for a chocolate chip cookie at 7:20 a.m. and Ben thinking pulling every book in the bookcase is a grand game, took upward of 45 minutes. I was supposed to do the routine three days a week. I averaged twice, and some times only got to it once.

But even my C+ effort paid off significantly. My lower back became less achy. My stomach, while far from a six-pack, at least stopped being tattooed daily by my belt. Lifting and carrying my tugboats — I mean children — didn’t feel like such a chore. Overall, I felt much more stable and capable: like my limbs could actually work as a team, not free agents.

But then, post-marathon, I retired the planks. I was burnt out on basement routines and training in general. I soon discovered that was a big mistake, as I was quickly back to an aching back and sloppy abs. Which, as a short sidenote, I gotta admit, I pretty much hate about exercise. It has no savings account. You either make regular deposits in the strength and cardiovascular accounts, or your balance quickly falls to zero. Sure, it’s easier to regain strength and aerobic fitness if your muscles already know the drill, but anything over a week or two off, and your body pulls the who-me? act, commonly seen in children asked to do chores: They moan and groan and pretend like they can’t remember how it’s done.

But if you’re like me, in your late 30s, this is no time to baby your body. As far as strength training goes, it’s do-or-die time. According to Wayne Westcott, Ph.D., a senior fitness executive at the South Shore YMCA in Massachusetts and exercise science wizard, you lose, on average, six pounds of muscle every decade of adult life.

Lean muscle is the worker bee of your metabolic system: It burns calories all of the time, and revs up during and after strength workouts. The more muscle you have, the faster your body processes the grilled cheese crusts you ate for lunch. And muscle also supports your bones and posture, which, as I know from spending too much time in grocery stores, can wither with age, leaving old ladies, struck with osteoporosis, hunched and forever staring at their Velcro-close sneakers.

Unfortunately, running alone doesn’t build muscle; in fact, says Wescott, a decade-long University of Florida study on masters runners found they lost five pounds of muscle during the 10 years.

Find time to squeeze in 20 minutes, two or three times a week, and I promise it will make a difference in your running, your health, your daily life.

Dimity McDowell writes and runs in Denver, where she lives with her husband and two children.

Crab walk: Put an exercise band around both ankles, then squat down to about 30 degrees, and step sideways across the floor. Builds the gluteus medius, a really important (and neglected) stabilizing muscle.

UPPER BODY/CORE

Back extensions on a stability ball: Rest your knees or toes on the floor and your stomach on the ball, and lift your back.

Or on the floor: Lie on your stomach, and raise your arms and legs at the same time, a la Superman.

Plank: Rest on elbows and toes, back straight, butt down. Super hard, but over quickly and very effective.

Side planks: Rest on left elbow, outer side of left foot, right foot stacked on left foot.

Crunches: Contract your abs on a stability ball, on the floor, like you’re riding a bike, twisting side to side. The options are endless.

More in News

Russian officials had disdainful words Saturday for a U.S. indictment that charged 13 Russians with interfering in the 2016 presidential election. Children’s stories, the plot of a preposterous Hollywood movie and “just blabber” were a few of the glib analogies they pressed into service.

President Donald Trump’s national security adviser said Saturday there was “incontrovertible” evidence of a Russian plot to disrupt the 2016 U.S. election, a blunt statement that shows how significantly the new criminal charges leveled by an American investigator have upended the political debate over his inquiry.

The University of Colorado leadership is grappling with how to address a nationwide nosedive in the favorability of higher education — particularly, among conservatives — as CU’s own representatives and decision-makers disagree on what’s behind the downturn.